A cube cannot have volume in inches. It can only have volume in cubic inches.

Cube of 3 inches per side.
Volume = 3*3*3 = 27 cu.in. <----answer.

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"3 inches on all its sides"
If perchance you mean the total length of all the 12 edges of the cube is 3 inches, then one side is 3/12 of an inch.
Volume then = (3/12)(3/12)(3/12) = 27/1728 cu.in.